Osmolal or Osmolar Gap

Definitions

Osmolarity = the total number of particles in solution per liter

Osmolality = the number of particles per kilogram of solvent

Background

Serum osmolality should be measured by the freezing-point depression method rather than by the vapor pressure method, since the latter technique will underestimate the concentrations of volatile alcohols (methanol, ethylene glycol, and isopropanol).

Calculations

Calculated Osm

Calculated Osm = 2 x Na + (glu/18) + (BUN/2.8) + (ethanol/4.6) + (methanol/2.6) + (ethylene glycol/5) + (acetone/5.5) + (isopropanol/5.9)

• Normal = 275-295

Osmolal Gap

Osmolal Gap = measured osm - calculated osm

(Normal = (-)14-10 mOsm/L)

DDX

Increased)

1. Alcohols
   1. methanol
   2. ethylene glycol
   3. isopropanol
2. Sugar
   1. glycerol
   2. manitol
3. Ketones
   1. acetone

See Also

Anion Gap and Osmolar Gap (High)